Davide Baldan

Sexual conflict affects the amount and duration of offspring care each parent invests, resulting in multiple parental care strategies sometimes coexisting within a single population. Understanding the persistence of multiple parental care strategies requires a precise estimate of the benefits and costs associated with parental decisions. Even though the benefits of brood desertion are well known, the reproductive costs of desertion ( i.e., nestlings' physiological conditions and survival), are less explored. We use rock sparrows, Petronia petronia , a species in which both uniparental and biparental care occur in the same population, to investigate the costs of brood desertion. Specifically, we continuously monitored breeding attempts to explore the behavioural mechanisms (desertion decision and compensatory responses) and the reproductive and physiological consequences (offspring corticosterone concentrations, oxidative stress, telomere attrition) of parental care strategies. We show that male desertion was not related to the initial value of the brood (clutch size, brood size) but was associated with a reduction in the survival probability of the nestlings. Females caring alone increased their per capita feeding rate, partially compensating for the lack of male care. Nestlings deserted earlier also experienced higher oxidative stress and had higher corticosterone concentrations during the early stages of development, but these effects did not persist to fledging, and there were no differences in telomere attrition. Our findings indicate combined reproductive and physiological costs associated with brood desertion. Considering these costs is essential to understand the evolution and persistence of polymorphic patterns of care. Read the free Plain Language Summary for this article on the Journal blog.

Parental cooperation is not self-evident, as conflicts often arise over individual contributions. Evolutionary game theory suggests this conflict may be resolved through negotiation, where parents adjust their care level based on their partner’s contribution. However, mathematical negotiation models typically predict low parental cooperation. As these models are not dynamically explicit and mostly neglect stochasticity, we employ individual-based simulations to investigate how parental negotiation strategies evolve and shape care patterns. Our results differ markedly from earlier analytical predictions. Parental negotiation strategies readily evolve, resulting in four alternative care patterns: uniparental care, sex-biased care and two types of egalitarian biparental care. Effective cooperation evolves regularly but, contrary to common expectations, always relies on a Tit-for-Tat strategy rather than parental compensation. Our study underscores that diverse cooperative patterns in animals can emerge from sex-specific negotiation strategies, even in the absence of initial sex roles and environmental variation.

Avian chronic hemoparasite infections occur commonly in wild birds, causing adverse effects on host fitness and breeding success. However, the potential impact of such infections on the incubation behavior has been scarcely experimentally studied. We reduced the infection of hemoparasites in wild‐breeding female pied flycatchers ( Ficedula hypoleuca ) through medication with primaquine to test the possible effects on incubation patterns compared with non‐medicated control females. As predicted, medicated females significantly reduced their parasite infection compared to control females. This had a direct significant effect on the female behavior, as medicated females were able to have longer incubation sessions, while control females reduced the time devoted to each incubation session. In addition, females from both treatment groups spent less time incubating as incubation progressed, with control females showing a greater reduction. In contrast, the average length of recess sessions did not vary across treatment groups. Moreover, incubation sessions were more frequently interrupted when clutches were smaller. However, these changes had no apparent effects on immediate fitness. To our knowledge, this is the first study showing changes in individual incubation patterns in response to parasites in a wild‐bird population, adding to previous studies showing that blood parasites have detrimental effects on bird reproductive success.

Global climate change has increased average environmental temperatures world-wide, simultaneously intensifying temperature variability and extremes. Growing numbers of studies have documented phenological, behavioural and morphological responses to climate change in wild populations. As systemic signals, hormones can contribute to orchestrating many of these phenotypic changes. Yet little is known about whether mechanisms like hormonal flexibility (reversible changes in hormone concentrations) facilitate or limit the ability of individuals, populations and species to cope with a changing climate. In this perspective, we discuss different mechanisms by which hormonal flexibility, primarily in glucocorticoids, could promote versus hinder evolutionary adaptation to changing temperature regimes. We focus on temperature because it is a key gradient influenced by climate change, it is easy to quantify, and its links to hormones are well established. We argue that reaction norm studies that connect individual responses to population-level and species-wide patterns will be critical for making progress in this field. We also develop a case study on urban heat islands, where several key questions regarding hormonal flexibility and adaptation to climate change can be addressed. Understanding the mechanisms that allow animals to cope when conditions become more challenging will help in predicting which populations are vulnerable to ongoing climate change. This article is part of the theme issue ‘Endocrine responses to environmental variation: conceptual approaches and recent developments’.

Theoretical models indicate that the evolution of biparental care depends on how parents behaviourally negotiate their level of care in response to those of their partner and whether sexes and individuals consistently vary in their response (compensatory response). While the compensatory response has been widely investigated empirically, its repeatability has rarely been assessed. In this study, we used a reaction norm approach to investigate the repeatability of the compensatory offspring provisioning of a parent after temporary removal of its partner in the pied flycatcher ( Ficedula hypoleuca ) across different breeding seasons and partners. We found that only females partially compensated for the short-term removal of the partner and their response was significantly repeatable across years while breeding with different partners. This study highlights the importance of considering among individual differences in negotiation rules to better understand the role of negotiation mechanisms in the evolution of parental care strategies.

For parents, rearing offspring together is far from a purely cooperative exercise, as a conflict of interest (‘sexual conflict’) exists over their optimum level of care. Recent theory emphasizes that sexual conflict can be evolutionarily resolved, and complete parental cooperation can occur, if parents directly respond (‘negotiate’) to each other and coordinate their level of care. Despite numerous experiments showing that parents are responsive to each other, we still lack empirical evidence of the behavioural mechanisms by which this negotiation occurs. In this study, we investigated the spatio‐temporal coordination of parental provisioning behaviour as a possible mechanism of negotiation over parental care. We deployed an automated radiotracking technology to track the provisioning activity of wild great tit Parus major pairs during chick rearing. Our analyses represent the first detailed spatial and temporal description of foraging coordination in songbird parents in a natural context. We demonstrate that the foraging behaviour of the two parents is highly coordinated in space and time, with parents changing their foraging locations in conjunction with their partners' movements. Therefore, foraging coordination could be a mechanism by which parents directly monitor and respond to each other's level of investment.

For parents, rearing offspring together is far from a purely cooperative exercise, as a conflict of interest (‘sexual conflict’) exists over their optimum level of care. Recent theory emphasises that sexual conflict can be evolutionarily resolved, and complete parental cooperation can occur if parents directly respond (‘negotiate’) to each other and coordinate their level of care. Despite numerous experiments show that parents are responsive to each other, we still lack empirical evidence of the behavioural mechanisms by which this negotiation occurs. In this study, we investigated the spatio-temporal coordination of parental provisioning behaviour as a possible mechanism of negotiation over parental care. We deployed an automated radio-tracking technology to track the provisioning activity of wild great tit ( Parus major ) pairs during chick rearing. Our analyses represent the first detailed spatial and temporal description of foraging coordination in songbird parents in a natural context. We demonstrate that the foraging behaviour of the two parents is highly coordinated in space and time, with parents changing their foraging locations in conjunction with their partners’ movements. Therefore, foraging coordination could be a mechanism by which parents directly monitor and respond to each other’s level of investment.

Quantifying organismal capacity for compensatory mechanisms is essential to forecast responses to environmental change. Despite accumulating evidence for individual variation in physiological plasticity, the causes and consequences of this variation remain unclear. An outstanding question is whether individual reaction norms are consistent across different environmental challenges, i.e. whether an individual that is responsive to one environmental variable will be equally responsive to a different environmental variable. Additionally, are these reaction norms themselves consistent over time, i.e. repeatable? Here, we quantified individual baseline glucocorticoid responses in house sparrows, Passer domesticus, to sequential manipulations of temperature, wind speed and food unpredictability that were repeated in discrete blocks of sampling under both control and stressor-exposed conditions. Individuals significantly decreased their baseline corticosterone levels and increased their mass during treatment exposure. This response was consistent across environmental challenge types. There was high repeatability in the intercept and slope of the baseline corticosterone reaction norm between environmental challenges but broad credible intervals in the repeatability of the reaction norm slope, suggesting that although glucocorticoid levels during baseline conditions are repeatable, among-individual variation in the shape of the glucocorticoid response may be higher than within-individual variation. Within-subject variation in baseline corticosterone levels was mainly explained by within-individual variation in body mass during stressor exposure. Despite the high lability in physiological traits, endocrine plasticity is repeatable across environmental challenges and may be able to evolve as a result of genetic accommodation, in which selection acts on genetic variation of reaction norms.